This invention is directed to the use of a highly efficient very-low temperature mixed refrigerant system with rapid cool down.
Refrigeration systems have been in existence since the early 1900s, when reliable sealed refrigeration systems were developed. Since that time, improvements in refrigeration technology have proven their utility in both residential and industrial settings. In particular, low-temperature refrigeration systems currently provide essential industrial functions in biomedical applications, cryoelectronics, coating operations, and semiconductor manufacturing applications. Conventional refrigeration systems have historically utilized chlorinated refrigerants, which have been determined to be detrimental to the environment and are known to contribute to ozone depletion. Thus, increasingly restrictive environmental regulations have driven the refrigeration industry away from chlorinated fluorocarbons (CFCs) to hydrochlorinated fluorocarbons (HCFCs), and more recently, to hydroflourocarbons (HFCs) due to a European Union law banning the use of HCFCs in refrigeration systems as of Jan. 1, 2001.
Traditional refrigeration systems that achieve very-low temperature cooling (xe2x88x9250 C. and xe2x88x92200 C.) using a single compressor include the Missimer, Kleemenko, and single expansion device varieties. Each of these systems employs refrigerant mixtures that are either flammable or chlorinated. The use of chlorinated refrigerants is increasingly restricted and, while flammable refrigerants are environmentally safe, they pose certain fire risks that make their inclusion into a system less desirable. What is needed is a way to achieve very-low temperature cooling using a nonflammable and nonchlorinated refrigerant mixture.
Providing refrigeration at temperatures below 223 K. (xe2x88x9250 C.) have many important applications, especially in industrial manufacturing and test applications. This invention relates to refrigeration systems which provide refrigeration at temperatures between 223 K. and 73 K. (xe2x88x9250 C. and xe2x88x92200 C.). The temperatures encompassed in this range are variously referred to as low, ultra low and cryogenic. For purposes of this Patent the term xe2x80x9cvery lowxe2x80x9d or very low temperature will be used to mean the temperature range of 223 K. and 73 K. (xe2x88x9250 C. and xe2x88x92200 C.).
There are many vacuum processes that have the need for such very low temperature cooling. The chief use is to provide water vapor cryopumping for vacuum systems. The very low temperature surface captures and holds water vapor molecules at a much higher rate than they are released. The net effect is to quickly and significantly lower the chamber""s water vapor partial pressure. This process of water vapor cryopumping is very useful for many physical vapor deposition processes in the vacuum coating industry for electronic storage media, optical reflectors, metallized parts, etc.
Another application involves thermal radiation shielding. In this application large panels are cooled to very low temperatures. These cooled panels intercept radiant heat from vacuum chamber surfaces and heaters. This can reduce the heat load on surfaces being cooled to lower temperatures than the panels. Yet another application is the removal of heat from objects being manufactured. In some applications the object is an aluminum disc for a computer hard drive, a silicon wafer for an integrated circuit, or the material such as glass or plastic for a flat panel display. In these cases the very low temperature provides a means for removing heat from these objects more rapidly, even though the object""s final temperature at the end of the process step may be higher than room temperature. Further, some applications involving, hard disc drive media, silicon wafers, or flat panel display material, involve the deposition of material onto these objects. In such cases heat is released from the object as a result of the deposition and this heat must be removed while maintaining the object within prescribed temperatures. Cooling a surface like a platen is the typical means of removing heat from such objects. In all these cases an interface between the refrigeration system and the object to be cooled is proceeding in the evaporator where the refrigerant is removing heat from the object at very low temperatures.
In traditional refrigeration systems, temperatures colder than xe2x88x9250xc2x0 C. are usually achieved by cascade refrigeration. Cascade systems require at least two compressors, each with separate refrigeration loops, with the evaporator of the warmer stage used to condense the refrigerant of the cooler stage. The lower temperature cooling achieved with these systems unfortunately is coupled with a greater power demand and, subsequently, a loss in system efficiency, as efficiency is directly proportional to the amount of heat removed from a substance and inversely proportional to the amount of power input to the system. What is needed is a way to improve the efficiency of a very-low temperature refrigeration system.
Industrial applications that require very-low temperature cooling often specify using liquid coolants. A common characteristic of these coolants is that they become highly viscous at such temperatures. Increased viscosity at lower temperatures is a common characteristic of most liquids. These particular liquids tend to be very viscous. As a liquid coolant is pumped through a closed loop system, the pressure drop experienced by the coolant as it flows through the evaporator affects the heat load on the refrigeration system, as higher coolant pressure drop requires higher input powers. Higher input powers to achieve a given amount of heat removal results in lower efficiency. What is needed is a way to achieve very-low temperature cooling without a high coolant pressure drop.
When designing refrigeration systems for a customer, the customer dictates certain parameters that must be met in order to fit the applications for which the system is intended. One such parameter is rapid cool down. When the customer demands that a system achieve the desired heat removal within 30 minutes of initiating the system, a refrigeration system with rapid cool down capabilities is required. Additionally, the physical size of the refrigerant evaporator has a direct effect on its cool down rate. The greater the mass of copper material that is present within a refrigeration system the more time is needed for the system to reach steady state refrigeration during cool down. Therefore, what is needed is a way to achieve rapid cool down in large refrigeration systems.
Certain applications find it desirable to control the coldest temperature supplied to a liquid coolant, as many industrial coolants become highly viscous and may even freeze out which makes it difficult or impossible to pump the liquids. What is needed is a way to control the coldest temperature supplied by a refrigeration system to a coolant.
In semiconductor applications many aspects of a refrigeration product are tightly constrained. Typically, limitations are placed on system floor space requirements, system height, input power, and cost. The ability to produce a system that meets all of these limitations is not obvious. For example, providing a short cool down time may be readily achieved with a large refrigeration system. However, such a system will require more input power and more floor space than allowed. Similarly, a fast cool down time could be enabled by a very compact heat exchanger which would produce a high pressure drop and would in turn increase the thermal load on the system (due to increased pump work on the coolant) and require excessive input power. Therefore, being able to meet all of the many performance requirements is difficult.
U.S. Pat. No. 6,112,534, xe2x80x9cRefrigeration and heating cycle system and method,xe2x80x9d assigned to Carrier Corporation (Syracuse, N. Y.), describes an improved refrigeration system and heating/defrost cycle. The system, for heating circulating air and defrosting an enclosed area, includes a refrigerant, an evaporator using said refrigerant for heating the circulating air; and a compressor for receiving the refrigerant from the evaporator and compressing the refrigerant to a higher temperature and pressure. Advantageously, the system further includes the combination of an expansion valve positioned between the compressor and the evaporator for forming a partially expanded refrigerant, a controller for sensing system parameters, and a mechanism responsive to said controller, based on the sensed parameters, for increasing temperature differential between the refrigerant and the circulating air, for improving system efficiency and for optimizing system capacity during heating and defrost cycles.
U.S. Pat. No. 6,089,033, xe2x80x9cHigh-speed evaporator defrost system,xe2x80x9d assigned to Dube, Serge (Quebec, Canada), describes a high-speed evaporator defrost system comprised of a defrost conduit circuit connected to the discharge line of one or more compressors and back to the suction header through an auxiliary reservoir capable of storing the entire refrigerant load of the refrigeration system. Auxiliary reservoir is at low pressure and is automatically flushed into the main reservoir when liquid refrigerant accumulates to a predetermined level. The auxiliary reservoir of the defrost circuit creates a pressure differential across the refrigeration coil of the evaporators sufficient to accelerate the hot high pressure refrigerant gas in the discharge line through the refrigeration coil of the evaporator to quickly defrost the refrigeration coil even at low compressor head pressures and wherein the pressure differential across the coil is in the range of from about 30 psi to 200 psi
U.S. Pat. No. 6,076,372, xe2x80x9cVariable load refrigeration system particularly for cryogenic temperatures,xe2x80x9d assigned to Praxair Technology, Inc. (Danbury, Conn.), describes a method for generating refrigeration, especially over a wide temperature range including cryogenic temperatures, wherein a non-toxic, non-flammable and low or non-ozone-depleting mixture is formed from defined components and maintained in variable load form through compression, cooling, expansion, and warming steps in a refrigeration cycle.
U.S. Pat. No. 5,749,243, xe2x80x9cLow-temperature refrigeration system with precise temperature control,xe2x80x9d assigned to Redstone Engineering (Carbondale, Colo.), describes a low-temperature refrigeration system for accurately maintaining an instrument with a time varying heat output at a substantially constant predetermined cryogenic temperature. The refrigeration system controls the temperature of the instrument by accurately adjusting the pressure of coolant at a heat exchanger interface associated with the instrument. The pressure and flow of coolant is adjusted through the use of one or two circulation loops and/or a non-mechanical flow regulator including a heater. The refrigeration system further provides a thermal capacitor that allows for variation of the cooling output of the system relative to a cooling output provided by a cooling source.
U.S. Pat. No. 5,396,777, xe2x80x9cDefrost controller,xe2x80x9d assigned to General Cryogenics Incorporated (Dallas, Tex.), describes a method and apparatus to refrigerate air in a compartment wherein liquid CO2 is delivered through a first primary heat exchanger such that sufficient heat is absorbed to evaporate the liquid carbon dioxide to form pressurized vapor. The pressurized vapor is heated in a gas-fired heater to prevent solidification of the pressurized carbon dioxide when it is depressurized to provide isentropic expansion of the vapor through pneumatically driven fan motors into a secondary heat exchanger. Orifices in inlets to the fan motors and solenoid valves in flow lines to the fan motors keep the vapor pressurized while the heater supplies sufficient heat to prevent solidification when the CO2 vapor expands through the motors. CO2 vapor is routed from the second heat exchanger to chill surfaces in a dehumidifier to condense moisture from a stream of air before it flows to the heat exchangers.
The present invention is a refrigeration system that uses a nonflammable, nonchlorinated refrigerant mixture to achieve very-low temperatures using a single compressor. The system is characterized by both high efficiency and rapid cool down time.